(1) Field of the Invention
This invention relates to plasma arc welding (PAW), and more particularly to plasma arc welding apparatus wherein an external constricting force is applied to the arc column to concentrate and focus it to maintain a dense column of energy.
(2) Description of Related Art
Plasma arc welding is a process wherein coalescence of metals is produced by heating them with a constricted arc between a nonconsumable refractory electrode and the work or between the electrode and a constricting nozzle having an orifice. A filler metal may or may not be added during the process. Thus, plasma arc welding is a method of electrical arc welding in a protective gas atmosphere wherein the arc is maintained between the workpiece and a nonconsumable electrode of a high melting point metal such as tungsten, the plasma of the arc being passed through a nozzle. The electrode is disposed within and insulated from a welding torch body and a front end having the constricting nozzle. An inert gas is supplied to the torch body and is channeled about the electrode exiting through the orifice. This gas acts upon the electric arc to constrict its shape to that of a narrow column and becomes ionized in the arc to form the plasma. In addition the orifice gas provides some shielding effect to the molten weld zone and the electrode and acts on the arc to constrict its shape and shield the weld. In most, if not substantially all cases, an auxiliary shielding gas is provided by channeling a second inert gas stream to blanket the area of arc plasma impingement on the workpiece to avoid contamination of the weld pool. An example of plasma arc welding apparatus is illustrated in U.S. Pat. No. 3,612,807.
One of the main advantages of the plasma arc is the directional stability and focusing effect brought about by arc constriction. This constriction narrows the arc column into an extremely dense energy source. However, upon exiting the orifice, the arc column begins to flare and become larger in diameter due to the absence of a constricting mechanism. This effects a decrease in the energy density of the arc prior to contacting the workpiece and thus reduces the effectiveness and efficiency of the process.